A power plant may include thermal machines, e.g. internal or external combustion engines, like gas turbine engines or reciprocating engines or others.
In all the above implementations the power plant includes, upstream from the thermal machine, an inlet air filtering system for removing water and/or dust and other impurities from the inlet air which, after filtration, is supplied to the thermal machine for combustion and/or ventilation purposes. The inlet-air filtering system normally comprises an upstream inlet air filter device for separating water from the inlet air and downstream filtration modules for removing dust and other impurities.
In humid weather conditions, for example rain or fog, the upstream air filter device separates water from the inlet air in order to not let droplets of water form on the downstream filtration modules, thus causing the latter to clog or, when frost conditions are reached, to ice. Filter clogging or icing may cause an excessive pressure drop in the downstream filtration modules, which can result in a significant reduction of the downstream thermal machine performance. In particular, if a thermal machine includes a compressor, which is typical in gas turbine engines, the pressure drop in the downstream filtration modules may cause such compressor to surge or security systems to stop the machine.
In dry weather conditions, on the other hand, when the inlet air does not include moisture, an upstream air filter device is useless, but also, potentially dangerous as it can itself cause undesired pressure drops. In such conditions, the air filter device should be removed or bypassed. Also in cold humid weather conditions, when ice forming on the inlet air filter device makes the pressure drop therein rise excessively, removing or bypassing the upstream inlet air filter device is required not to cause the shutdown of the downstream thermal machine, at least as long as ice does not obstruct significantly the downstream filtration modules.
In known inlet air filtering systems, the upstream air filter device is removed manually when water separation is not needed or desired. Such a solution may require the shutdown of any downstream thermal machine during the removal operations and is therefore scarcely efficient and, in addition, time demanding for field operators.
In other more efficient solutions, for example in the one described in U.S. 2011/0083419, there is provided an inlet air filtering system where the removal of the upstream water separation devices is made automatically by a remotely operated actuator. In U.S. 2011/0083419 a filter bypass assembly including a water separation filter is described. The bypass assembly is moved by an actuator between a first operating position, which intercepts the inlet air flowing in the filtering system, and a second bypass position, which lets the inlet air to reach the downstream filtering modules without crossing the water separation filter.
The latter solution is clearly an improvement over manually operated solutions, however it shows a plurality of inconveniences.
Firstly, when in the bypass position, even if the bypass assembly is not crossed by the inlet air flow, it is not isolated by any impurity which might be present in the air and therefore might deposit on or damage the water separation filter. In particular, this could occur in sandy desert environment, during sand storms.
Secondly, when in the bypass position, the bypass assembly is not isolated from the temperature and humidity conditions of the inlet air, and therefore, if the frost condition was reached causing ice to form on the water separation filter, this could be de-iced only passively, after temperature and humidity conditions of the inlet air will return above the frost point and remain in such condition for a convenient time interval. Thirdly, the bypass assembly could be improved in order to reduce its mass and simplify the cinematic elements, e.g. hinges and actuators, which are required to move the bypass assembly. Optionally, such simplification could lead to a manually operated bypass assembly.
Fourthly, also control strategies could be improved. In U.S. 2011/0083419 the bypass element is only controlled through temperature measurement. It could be envisaged to add humidity measurements in order to better operate the bypass element, following weather conditions.